In particular, allow longer multistream select reads
* clear secrets explicitly * simplify keygen * avoid some trivial memory allocations * fix little endian encoding of nonce
* remove readLoop in secure protocols, fix security issues * fix Defect on remote sending 0-byte noise/secio message * remove msglen from `write` (unused) * simplify SecureConn data flow * document some control-flow issues * unify exception behaviour across noise and secio * secio would not raise on mac/decryption errors * fix compile error